Question

Sulfur hexafluoride and Sulfur tetrafluoride are two common halides of sulfur.

a) Draw their lewis dot structures and describe their molecular shapes including bond angles.

b) Write a balanced reaction for the synthesis of SF₆.

c) Describe in detail the differences in reactivity and chemistry of the observed by the two compounds.

d) Please describe in as much detail as possible, uses for SF₆.

Answer 1

b) SF₆ is synthesized by reaction of elemental sulfur and fluorine

S + 3 F₂ ---> SF₆

The reaction actually yields a mixture of products which need to be purified to isolate SF6.

c) SF₆ and SF₄ have very different reactivities.

SF6 is unreactive and its reaction chemistry is virtually non-existent. Its unreactivity is attributed to high bond energy of S-F bond. Also the S atom is surrounded by six F atoms and well shielded from attack by other reacting molecules.

SF4 on the other hand is very reactive. this is due to presence of a lone pair of electrons on sulfur and also to the fact that in this molecule S is surrounded by only four F atoms. Thus it can be attacked by other molecules resulting in a reaction. SF4 reacts with water to give SO2. It is also used as a fluorinating agent.

d) (i) SF6 is a dielectric material and a very good electrical insulator. Its high dielectric constant (double than that of air), non-reactive, non-corrosive nature makes it very useful as an insulating gas in electrical transmission and distribution equipment such as circuit breakers, high voltage transformers, switchgears and generators.

(ii) It has also been used to provide an inert atmosphere in the manufacture and casting aluminium and magnesium. As SF6 is much denser than air (almost six times), it forms a dense blanket that keeps atmospheric oxygen and nitrogen away from the hot metals during processing thereby preventing their reaction with O2 and even N2.

(iii) Due to the reduced velocity of sound in SF₆, it has been used in double-glazed windows for sound insulation.

(iv) SF6 is used as a tracer gas to check the efficiency of ventilation systems. One example is for checking the efficiency of fume-hoods. This is mainly because its a non-toxic gas whose concentration can be measured accurately at low concentrations and it is naturally present in negligible amounts in the Earth's atmosphere.

SF6 has now been recognised as a very potent Greenhouse gas and is now being replaced with other options as much as possible.